1. Field of the Invention
The present invention relates to a stabilized polyamide substrate and a process for making the same. More particularly, this invention relates to a fiber or article made from a polyamide polymer, preferably having a hindered amine light stabilizer chemically bound thereto during polymerization, and treated with an antioxidant and ultraviolet inhibitor in the presence of a substance that is a solvent for the antioxidant and ultraviolet inhibitor and is a swelling agent for the polyamide. The treated polyamide substrate, preferably dyed, has unusually high strength as compared with the untreated polyamide substrate, and is useful in devices, and/or fabrics covering same, that are subject to weathering or exposure to severe environmental conditions during normal use.
2. Brief Description of the Related Art
Dyed fabrics woven from organic polymer fibers such as polyamides have been used in flags, decorative banners and as covers for personal flotation devices, such as life preservers. Prolonged exposure to sunlight, commonly referred to as weathering, tends to fade the dyed polyamide material and to reduce its physical properties such as strength retention and resistance to elongation. This occurs when the dye and polymeric components of the material absorb damaging wavelengths of light.
Various organic light stabilizers are known for use with polymer substrates like films, fibers and articles manufactured therefrom. Hindered amine light stabilizers (HALS) have been found to be particularly effective organic light and/or heat stabilizers, with one particularly effective stabilizer being that set forth in U.S. Pat. No. 5,618,909 to Lofquist et al., hereby incorporated by reference. Lofquist et al. discloses an efficient, environmentally friendly process for preparing light stabilized polyamide substrates such as molded objects, films, fibers and fabrics, and in particular flags, decorative banners, and fabric covers for personal flotation devices, with a HALS chemically bound to the polyamide and thus no longer susceptible to migration, leaching, and/or volatilization during downstream processing. Although the disclosed HALS stabilizes the polyamide polymer composition against heat and light, fading and coloration problems still exist when the polymer is dyed and exposed to excessive heat, moisture and light conditions. Particularly affected are dyed polyamide polymers that are used in water safety and water recreational devices, such as life vests, floats, banners, and other such normally weathered devices or fabrics.
It is also known to stabilize polymers against ultraviolet and oxidative deterioration by incorporating benzotriazole derivative ultraviolet absorbers and hindered phenol antioxidants into bulk polymers during melt processing. U.S. Pat. Nos. 3,935,163 and 5,380,774 disclose numerous benzotriazole derivatives and hindered phenol antioxidants suitable for this purpose. The disclosures of U.S. Pat. Nos. 3,935,163 and 5,380,774 are hereby incorporated by reference to the extent not incompatible herewith.
However, it is not always practical to incorporate bulk additives in polymers prior to a fiber spinning operation. The stability of a fiber spinning operation is extremely sensitive to the rheological properties of the polymer. Additives must be compatible with the polymer, must not unduly influence the polymer rheology, and must be capable of being rigorously controlled to a constant concentration. Further, a bulk additive must be incorporated at high concentration in order to provide useful concentrations at the surface of the article where needed. These are serious limitations on allowable materials.
A need exists for an effective method of treating fibers and articles after spinning or melt processing to improve strength retention after exposure to light. Ideally, this method should provide the active constituents at the surfaces of the fibers and be practicable in conjunction with dyeing so that no additional steps are required in manufacturing.
The present invention satisfies these needs. It provides means of improving the strength retention of a polyamide substrate after exposure to light. More particularly, the invention improves the strength retention of fibers and articles by treatment after spinning or melt processing. The method provides the active constituents at the surfaces of fibers and is practicable in conjunction with conventional fiber dyeing.
Determination of effective strength of the substrate, and resulting fibers, results from Weatherometer measurements of the substrate in question after exposure to light. Although desirable, it has proven particularly difficult to manufacture a polyamide substrate possessing a high breaking strength after light exposure, particularly a breaking strength of greater than about 300 Newtons after 300 hours of weathering [Underwriters Laboratories Canada, 300 hours of fading ASTM G26 Xenon Arc and ASTM D5034 Break Strength (Grab Test)]. The present invention achieves this superior breaking strength for polyamide fabric after light exposure.
The present invention includes a process for preparing a polyamide substrate characterized by high strength, even after extended exposure to light. The process comprises the steps of treating a polyamide substrate, preferably dyed, with an effective amount of an ultraviolet inhibitor and an antioxidant in the presence of a substance that is a solvent for the ultraviolet inhibitor and the antioxidant and is a swelling agent for the polyamide, to thereby impart enhanced breaking strength to the polyamide substrate after exposure to light.
An article made by this process is also within the scope of the present invention, e.g., a film, fiber or molded article, as well as products made therefrom, such as fabrics and fibrous networks. Life preservers and other buoyant device products, as well as fabric covers therefor, are examples of such articles.
The present invention further includes the fabric comprising a polyamide fibrous substrate treated with an ultraviolet inhibitor and an antioxidant, and optionally but preferred, with an acid dye.
The preferred polyamide substrate comprises a heat and/or light stabilizing amount, preferably from about 0.1 to about 1.0, more preferably 0.2 to 0.5 weight percent, of a hindered amine bound thereto by reaction of a functional group of the hindered amine with an end group of a precursor of the polyamide. The preferred hindered amine is represented by the formula: 
in which
R1 comprises the functional group, which is an amine or an amide-forming group:
R2 is alkyl, preferably methyl or ethyl; and
R3 is selected from the group consisting of hydrogen; alkyl of 1 to 3 carbons; and xe2x80x94OR4 in which R4 is selected from the group consisting of hydrogen, methyl, and alkyl of 1 to 7 carbons.
When the treated polyamide substrate of the present invention is exposed to sunlight, its useful life is greatly increased compared to untreated polyamide substrates. This is particularly applicable to polyamide fibers and fabrics made therefrom, especially nylon 6 fibers and fabrics.